Clifs

- introduce `plugin_description` and `plugin_summary` class attributes as an option to
define cli help texts for plugins.
- provide plugin descriptions for all included plugins when calling `clifs {plugin} --help`
- fix some help texts

- streaming editor: nicer reporting

- add streaming editor plugin (`sed`)
- rename:
- highlight regex matches in the reporting
- cleaner reports
- delete:
- cleaner reports

- nicer help texts:
- show default values for cli arguments
- do not show destination variable for positional cli arguments

- backup:
- catch scenario where no parameters specifying source and destination directories are provided
- fix regex example in README.md
- minor changes in docu

- path_getter: allow to filter processed files and folders by last modification (mtime) and/or creation/change date. Introduced cli options are: 'mtime_stamp_older', 'mtime_stamp_newer', 'ctime_stamp_older', 'ctime_stamp_newer'.
Plugins supporting time filtering: `ren`, `cp`, `mv`, `del`
- some wording in the docu